The mass of LSTs and LCTs off Normandy came from a buildup that began in 1940 for the 1944 assault. Four years of preparation undercut claims that the Allies could simply have mounted D-Day much earlier for political or humanitarian reasons. Ship construction was only one requirement: crews had to learn controlled beaching, unloading, retraction, and repeated cross-Channel runs without disabling their craft. The assault wave could establish a foothold, but the Normandy campaign and advance beyond it depended on successive loads of troops, vehicles, and supplies. Untrained crews might reach the beach once and then damage enough vessels to stop the follow-up traffic on which the campaign depended.
Chapter · 25:09-29:04
D-Day required four years of amphibious preparation
The landing fleet, trained crews, and repeated supply waves explain why the Normandy invasion could not simply have been launched years earlier.